A turbomachine transfers energy between a rotor and a fluid. The turbomachine according to the invention includes electromagnetic parts that allow it to operate when supplied electricity as a pump or compressor, or when driven by passing fluid as a generator.
In practice, turbomachines having an axially, magnetically supported rotor shaft are known where for axial support the magnetic bearing halves are associated with an axial bearing disk or oppositely situated shaft shoulders that are shrunk onto the rotor shaft. Combination magnetic bearing designs are also known in which the two axial magnetic bearing halves are mounted on one end of the rotor shaft on the two side surfaces of a rotating multidisk assembly located on the rotor shaft, and the side edges of the multidisk assembly are enclosed by a radial magnetic bearing, the opposite end of the rotor shaft likewise being also supported on a radial magnetic bearing.
Also known in practice are turbomachines having the above-described features, and whose rotor shaft is supported on an active magnetic bearing mounted on the end of the rotor shaft axially outside the radial impeller. Active magnetic bearings require a continuous power supply, as well as so-called backup bearings for protecting the bearing in the event of a power failure. Such backup bearings are typically mechanical, radial, and axial emergency ball bearings that support the rotor shaft when at rest or in the event of a failure of the magnetic bearing.
Since for the known active magnetic bearings the required magnetic field is generated by use of electromagnets, the magnetic field and thus the acting bearing force may be modified by varying the current in the coils of the electromagnets. Therefore, to allow support of a rotor shaft on an active magnetic bearing, feedback control is necessary in order to adjust the corresponding bearing force. Such turbomachines may be operated at very high rotational speeds when using a magnetic bearing for the rotor shaft. However, the bending-critical natural frequencies of the rotor shaft at high operating speeds are in the vicinity of or below the maximum continuous operating frequency of the turbomachine. This makes stable, active regulation of the magnetic bearings very complicated or even impossible.
Operation of the turbomachine in a state in which frequencies may arise that are in the range of bending-critical natural frequencies of the rotor shaft is essentially precluded by means of two preventative measures. First, the permissible operating speeds have already been reduced in the design of the turbomachine. For a given output and demand on the turbomachine this measure results in the construction of larger turbomachines whose compressor or expander stages, which are designed below the optimal rotational speed, also have reduced efficiency compared to optimum. Second, an additional axial bearing disk may be omitted. In this manner the mass of the rotor shaft may be reduced, since the rotor shaft may be shortened by the length of the shaft segment for the additional axial bearing disk, thereby increasing the bending-critical natural frequencies of the rotor shaft. The axial magnetic bearing halves engage on much smaller shaft shoulders or on rotating multidisk assemblies shrunk onto the rotor shaft when the axial bearing disk is omitted, but this results in an undesired reduction in the axial bearing forces.